From the prior art it is known that functional monomers, such as hydrolyzable vinylsilanes, for example, help to improve the properties of polymer dispersions when used as binders in paints.
U.S. Pat. No. 3,729,438 describes lattices based on vinyl acetate-vinylsilane copolymers with a vinylsilane fraction of 0.5% to 1.0%. The lattices are prepared in a discontinuous emulsion polymerization process, in the presence of nonionic or anionic emulsifier, preferably of mixtures of nonionic and anionic emulsifiers.
DE-A 2148457 describes a process for preparing polymer dispersions containing silanol groups, with sharply improved wet adhesion of the architectural coating materials of the invention. DE-A 2148458 describes a process for preparing vinyl acetate-vinylsilane copolymers in a discontinuous emulsion polymerization process, in the presence of anionic, cationic and/or nonionic emulsifiers and/or protective colloids.
EP 0327006 A2 discloses copolymer dispersions containing 0.05%-2% by weight of monomer units of unsaturated hydrolyzable organic silicon compounds, which find application in low-emission emulsion paints, coating materials or dispersion renders. The aqueous dispersion is stabilized using hydroxyethylcellulose, nonionic emulsifier, and vinylsulfonate.
EP 327376 A2 describes vinyl ester copolymers with a vinylsilane fraction of up to 0.8% by weight as binders for paint compositions with high scrub resistance. They are prepared by discontinuous emulsion polymerization in the presence of emulsifiers and/or protective colloids.
EP 612771A1 describes aqueous polymeric dispersions for producing low-emission emulsion paints on the basis of copolymers which as functional monomer units comprise vinylsilane in combination with (meth)acrylamide and/or (meth)acrylic acid. They are prepared in a batch process in the presence of anionic emulsifiers or mixtures thereof with nonionic emulsifiers.
The report of the Fatipec Congress of Jun. 10-14, 1996, includes a study of the properties of VAE copolymer dispersions as binders in highly filled paints, particularly with regard to the copolymer used and the effect of the emulsifier-protective colloid system. Described as particularly advantageous are VAE copolymers containing <1% of vinylsilane units, prepared in the presence of protective colloid and nonionic emulsifier. An increase in the emulsifier content from 1.5% to 50 brought about a reduction in the particle size and in the Tg and led to an improvement in the pigment-binding capacity.
The key breakthrough in the development of binders for coating materials with high wet abrasion resistance was achieved with the VAE copolymers with vinylsilane- and epoxy-functional comonomer units in accordance with EP 1153979 A2. The preparation takes place by discontinuous emulsion polymerization in the presence of mixtures of nonionic and anionic emulsifiers.
To improve the hydrophobicity of such binders from EP 1153979 A2, EP 1308468 A1 proposes modifying them with polysiloxane units.
WO 2006/111290 A1 advises using coating materials with high scrub resistance on the basis of VAE copolymers with silane and epoxy monomer units, these binders being prepared with the sulfosuccinates and nonionic emulsifiers already described from EP 1153979 A2.
EP 2017313 A1 advises designing the preparation of binders for coating materials with high weathering resistance, on the basis, known from EP 1153979 A2, of vinyl ester polymers having silane-functional and epoxy-functional comonomer units, in such a way that in a first stage, vinyl ester and optionally further monomers are added together with emulsifier and/or protective colloid, and, after the polymerization reaction has subsided, further monomers are added and the polymerization is initiated again.
In EP 2166050 A2 an attempt is made to improve the wet abrasion resistance of the vinyl ester copolymers with silane-functional and epoxy-functional comonomer units, known from EP 1153979 A2, by means of blending with acrylate latex.
Vinyl acetate-ethylene copolymers which contain vinylsilane and epoxy monomer units are virtually impossible to improve in terms of wet abrasion resistance in application as coating materials. A problem, however, is their preparation in a discontinuous emulsion polymerization process, in particular because of the long cycle time of batch operations like that from EP 1153979 A2, or on account of the very costly and inconvenient, multistage processes from EP 2017313 A1 and EP 2166050A2.
As compared with the frequently employed batch or semi-batch operations, continuous polymerization processes offer the advantage of a more efficient utilization of plant capacity. Downtime as a result of cleaning and preparation phases, which have to be inserted between individual batch procedures, are taken into account hardly at all, allowing significantly high capacities and lower preparation costs to be achieved. A disadvantage of continuous polymerization, however, is that product properties change in comparison to the batch operation, in some cases disadvantageously so. It is known, for example, that the stabilization of continuously prepared dispersions is considered to be critical. Owing to a broad residence-time distribution, the dispersions obtained are usually broader in distribution, with coarser particles. For application in coating materials, the particle size and its distribution are important variables, which impact on performance properties, such as the wet abrasion resistance, for example, of paints enhanced with these binders. Shifting the particle size distribution into a larger range therefore results in a decrease in the wet abrasion resistance.
A favored plant configuration frequently selected for continuous polymerization is the stirred-tank cascade, since polymerization in stirred-tank reactors is known from batch operation and the cost and effort involved in converting a plant is comparatively small by comparison with other known continuous plants, such as a tube reactor, for example. The difficulty is in particular the establishment of suitable and stable operating conditions. Consequently, operational fluctuations and irregular product properties are not uncommon. Coatings on the reactor walls and an oscillating particle size are identified as consequences.
Petrocelli, Polym. Sci. Eng., 80, 544 discloses a process for continuous emulsion polymerization in which polymerization takes place in two serial pressure reactors. The monomers, polyvinyl alcohol, and seed latex are metered continuously to the first pressure reactor, and polymer dispersion is withdrawn in equal volume from the second pressure reactor. A disadvantage here, as well as the use of seed latex, is the increase in particle size relative to the batch process, since an increase in particle size in coating materials has the effect of lowering the wet abrasion resistance.
DE-A 2456576 describes a process for continuous polymerization of vinyl acetate-ethylene dispersions in a pressure reactor with a downstream reaction tube. In order to avoid wall accretions and to ensure a uniform course of the polymerization, the use is advised of a redox initiator mixture containing a 3- to 10-fold excess of reducing agent.
EP 5073 B1 discloses a process for continuous emulsion polymerization of vinyl acetate and ethylene, in which the reactants are provided in premixers and are transferred together with seed latex into a pressure reactor. The product from the pressure reactor is then transferred into a postpolymerization reactor.
EP 1067147 A2 describes a continuous polymerization process for producing high-solids-content vinyl acetate-ethylene copolymer dispersions. For the continuous polymerization, seed latex is used, and for stabilization polyvinyl alcohol exclusively is used, and no emulsifier is employed. The polymerization takes place in two serial pressure reactors and a devolatilizing reactor. Monomers, seed latex, and PVOH are added continuously in the 1st reactor.
EP 1174445 A1 describes a process for continuous polymerization of vinyl ester-ethylene copolymers where the reducing component of the redox initiator is metered at least partly in the first reactor. Relatively coarse polymer particles with a weight-average diameter of more than 1000 nm are obtained.
EP 1323752 A1 describes a process for the continuous polymerization of protective-colloid-stabilized polymers based on vinyl esters and ethylene. With polyvinyl alcohol as the protective colloid used, dispersions having a weight-average diameter Dw of 800 to 1400 nm are obtained, the protective colloid being metered into both reactors. This form of particle stabilization affords an inadequate particle size distribution for the preparation of dispersions for the application described here (binders in paints).